1. Field of the Invention
The present invention relates generally to display of information on a video monitor and, more particularly, is concerned with a quasi-resonant random access deflection system to enable a high performance calligraphic, or random access-type, display monitor to operate at power levels heretofore typical only of a raster scan, or resonant-type deflection, display monitor.
2. Description of the Prior Art
Television receivers and other CRT type devices typically use a beam of fast-moving, magnetically deflected electrons to produce a video picture. Two sets of electromagnetic windings are commonly used to achieve deflection of the electron beam. These sets of windings are assembled into a deflection yoke to produce vertical and horizontal magnetic fields which fluctuate with respect to time in order to deflect the electron beam in the horizontal and vertical directions. The beam may be made to impact anywhere on the screen of the display monitor by passing the correct amount of current through the deflection yoke.
Two basic types of systems for controlling current flow in the deflection yoke are in use at the present. Of the two, the more widely-used one is the raster scan resonant deflection system; such as used in the control of the conventional television receiver. The other, less widely-used one, is the vector scan random deflection system; commonly used in display monitors for computer aided design (CAD) and simulation equipment.
In the raster scan system, a sawtooth waveform of current is passed through the horizontal deflection windings causing the spot to scan from left to right across the screen of the display monitor and then retrace to the left and, similarly, another sawtooth waveform of current is passed through the vertical deflection windings causing the beam to scan from top to bottom and then retrace to the top. The combined action of the vertical and horizontal magnetic fields on the electron beam produces a frame of light on the screen called a raster.
The raster scan deflection system is highly efficient in terms of energy use. Scanning and retrace of the beam is mainly produced through exchange and recovery of a given quantity of energy between the shaper and flyback (or retrace) capacitors of a circuit oscillating in concert with the deflection yoke at their combined resonant frequency. Except for small ohmic losses in the deflection yoke windings and losses within the resonant switch of the system, a resonant amplifier merely controls the transfer of energy between the yoke's magnetic field and the electric field within the pair of capacitors. Even though several millijoules of energy are exchanged up to 256,000 times per second in a modern high performance display monitor, power dissipation in the resonant amplifier is typically only a few watts. Similarly, the current draw on the power supply of the system is usually only several hundred milliamperes whereas the peak deflection yoke current may be 5 to 10 amperes.
Since information must be provided which relates to each picture element, or pixel, as the electron beam is scanned during the production of each frame, the raster scan deflection system is inefficient in terms of the inordinately large amount of computation and storage which is required.
In the vector scan system, a control signal, either analog or digital, corresponding to the coordinates or vectors of the desired location of the electron beam on the screen of the display monitor is supplied by the user to X and Y inputs of the deflection amplifier. Thus, the electron beam, rather than being moved through a scan and retrace pattern as in the raster scan system, is moved and positioned at random on the screen.
The vector scan system is highly efficient in that only a small quantity of information must be provided to support calligraphic applications. Only a comparatively short list of vectors need be stored in memory to direct the electron beam to the correct screen positions.
On the other hand, random access deflection is conventionally accomplished with a linear-mode amplifier. The linear amplifier is used to produce the desired waveform to deflect the electron beam. In a high performance random access display monitor, peak deflection yoke currents and deflection voltages can easily reach 20 amperes and 75 volts respectively. A linear-mode random access deflection amplifier may therefore have to dissipate several orders of magnitude greater power than a resonant type amplifier.
Heretofore, the conventionally accepted practice has been to choose between the use of one or the other of the two types of electron beam deflection systems; the raster scan resonant system or the vector scan random system. Along with the advantages associated with the selected system, the user regrettably also had to accept its drawbacks. It was thought that no merging of the two systems was possible. No matter how desirable the advantages of the raster scan resonant deflection system are, informed opinion in the field has heretofore considered the system unadaptable to random access deflection.
A few words of explanation might help to understand this conventional thinking. The resonant deflection amplifier is not really an amplifier at all. It amplifies nothing. Rather, it controls the transfer of energy between the pair of capacitors (shaper and flyback) and the yoke windings, synchronously with the video signal modulating the electron beam. Thus, the conventional resonant deflection amplifier has heretofore not been considered suitable for random access deflection. As a random access deflection amplifier, it has two fundamental shortcomings. First, there is no mechanism by which the electron beam can be "parked". That is to say, the resonant deflection amplifier cannot position the electron beam; it can only scan the beam across the face of the display monitor screen. Second, right-to-left deflection in a resonant deflection amplifier is intrinsically different from left-to-right deflection.
Notwithstanding the weight of conventional authority, which maintains that these two types of deflection systems are not compatiable and counsels against any expectation of success in achieving a merger of their more desirable features into a hybrid-type deflection system suitable for calligraphic applications, it is perceived that a need still exists for a fresh design approach to accomplish just that, one having the objective of bridging the dichotomy between these two deflection systems.